1. Field of the Invention
This invention relates to a linear compensating circuit and more particularly, to a linear compensating circuit for compensating the linearity of an input-output characteristic of an amplifier.
2. Description of the Related Art
Generally, in a linear compensating circuit of a high frequency power amplifier, the output power of the amplifier is made equal in level to the input power thereof and compared therewith and then, controlled so that the difference i.e., variation, between the input and output powers of the amplifier can be made zero thereby keeping its input-output characteristic linear. An exemplary application of such a linear compensating circuit is described.
In the front stage of a high frequency power amplifier, an amplitude compensating circuit for compensating an amplitude of an input signal to be inputted to the amplifier is provided. In addition, there are provided a first detection circuit for detecting a level of the power of the input signal, a second detection circuit for detecting a level of the power of an output signal of the amplifier and a differential amplifier for detecting the difference in power between the input and output signals of the amplifier and for outputting a signal in accordance with the detected difference. The level of the output signal is made equal to that of the input signal (that is, the level before amplification) and then, sent to a second detection circuit. The amplitude compensating circuit controls the amplitude of the input signal in advance in accordance with the signal from the differential amplifier so that the difference in power between the input and output signals is made substantially zero.
As explained above, with the conventional linear compensating circuit, an input signal to be inputted to the amplifier is subjected to amplitude compensation and then, sent thereto, thus keeping the input-output power characteristic of the amplifier linear over the entire circuit including the linear compensating circuit and the amplifier.
According to the conventional linear compensating circuit as described above, the linearity of an input-output power characteristic of a high frequency power amplifier can be improved so that distortion of an output signal is prevented from being induced when the amplifier is to be operated in the non-linear range when the level of the input signal is high. With this linear compensating circuit, however, when the amplifier is being operated in the saturation range because the input signal level is further increased, an acceptable input-output characteristic cannot be obtained. Thus the a problem that a desired reduction in signal distortion be attained remains.
Thus, an object of this invention is to provide a linear compensating circuit capable of amplifying a signal with the extremely low level of signal distortion even when an amplifier is being operated in the saturation range.
Another object of this invention is to provide an amplifier capable of amplifying a signal while producing an extremely low level of signal distortion over a wide range of input signal levels.